In reply to strickie:
I'll have a stab at it:
6a to me means hard, hard to work out or just physcially demanding moves. It's probably going to feel like french 7a if it's at all sustained...
alright, alright, alright, French 6a on an indoor wall:
The wall I used to work at was about 11 metres high so these descriptions are for that height:
slab - can have some fairly technical moves and/or smallish crimps.
Vertical - can have some technical sidepull/undercling moves on minijugs or more straight foward climbing on large crimps.
Slight overhang - Line of minijugs.
overhang - Line of uberjugs.
more than 20 degree overhang - It's not 6a anymore with any holds.
Obviously as other people have pointed out, it's so subjective and variable from venue to venue that it's barely worth even trying for a description!